The invention broadly relates to reactive compositions containing superoxide ion, to methods for preparing those compositions and to methods for using them in degrading and detoxifying halogenated organic compounds.
The hazard to health and to the environment caused by synthetic halogen-containing organic chemicals is well understood. Compounds such as polychlorinated biphenyls (PCBs), dichlorodiphenyltrichloroethane (DDT), decachlorocatahydro-1,3,4-metheno-2H-cyclobuta [c,d]-pentalen-2-one(Kepone.RTM.), and 2,4,5-trichlorophenoxyacetic acid, (2,4,5-T), have been found to be persistent environmental poisons, and, therefore, to require a safe and effective means of disposal.
Halogenated organic compounds are difficult to degrade because of the highly stable nature of the carbonhalogen bonds present in those compounds. These compounds are not only resistant to biodegradation, they cannot be degraded in a practical and effective manner by any of the conventional chemical decomposition methods. In most cases, methods, such as chlorolysis, catalytic dehydrohalogenation, molten salt reactions, ozone reactions, and alkali metal reduction, do not achieve complete dehalogenation. Moreover, these prior art methods typically require expensive reagents, inert atmospheres, extensive temperature control, complex apparatus, and substantial energy consumption.
PCBs present a particularly serious disposal problem. Although PCBs were once widely used because of their excellent insulating properties, as dielectric fluids in electrical equipment such as transformers and capacitors, all such use has been banned by the U.S. Environmental Protection Agency (EPA), because it has been found that they accumulate in human fatty tissue and are extremely toxic. Incineration has been viewed as a practical method for achieving the complete decomposition of PCBs. Incineration of PCB-contaminated materials is wasteful, however, since potentially recyclable materials, such as dielectric and hydraulic fluids, which may contain a relatively small amount of PCBs, are destroyed in the process. To avoid such waste, it has been proposed to treat recyclable materials contaminated by PCBs with an absorbent, e.g., by passing the material through a bed of activated charcoal or a resin to selectively remove the PCBs from said material. The disposal of absorbed PCBs remains a difficult problem.
Significant quantities of waste-containing halogenated organic compounds are presently stored by manufacturers, processors, or consumers. These chemicals must be disposed of in an environmentally acceptable manner. Efforts to develop a safe, practical and effective process for their disposal are continuing. Often such wastes are either incinerated or are stored in dumps for toxic wastes. In the past, negligence in locating such dumps has had catastrophic consequences in exposing large populations to toxic compounds. As a result, massive clean-up efforts are being undertaken to degrade and detoxify these compounds.
Several chemical methods for decomposition of PCBs have been proposed. In one method, high surface sodium, sodium/naphthalene, and sodium naphthalide are employed. These methods are limited, however, because the reagents are difficult to prepare, expensive to ship and unstable in storage. Moreover, the sodium-containing decomposition reagents are sensitive to oxygen and to water and therefore cannot be used reliably. Other methods include those described in U.S. Pat. Nos. 4,400,552, 4,417,977, and 4,602,994, the so-called KPEG or NaPEG processes.
There is thus a clear need for an efficient and safe method to degrade toxic halogenated waste to harmless and environmentally compatible products. However, such a method has been elusive because of certain basic considerations with respect to the nature of toxic halogenated compounds. One aspect of the problem relates to the wide diversity of such compounds. The compounds range from such simple molecules as carbon tetrachloride and chloroform to complex insecticides such as p-p'dichlorodiphenyltrichloroethane (p-p' DDT).
Superoxide ion, which is an effective nucleophile, has proven particularly effective for destroying halogenated organic compounds such as polychlorinated biphenyls (PCBs) and similar toxic materials that create environmental hazards. U.S. Pat. Nos. 4,468,297 and 4,410,402 describe the use of superoxide ion for degrading halogenated organic compounds and halogenated olefinic hydrocarbons.
While halogenated hydrocarbons typically react slowly or incompletely with traditional bases, and this becomes an even more significant problem as the number of halogen atoms in the compound increases, superoxide ion overcomes this difficulty, and reacts rapidly with halogenated hydrocarbons when the reaction is carried out in an aprotic solvent.
Several methods have been developed to generate superoxide ion, including, e.g., pulse radiolysis of dioxygen, Gebicki et al., J. Am. Chem. Soc. 1982, 104, 796, photolysis of hydrogen peroxide in aqueous media, McDowell et al., Inorg. Chem. 1983, 22, 847 and base-induced decomposition of hydrogen peroxide, Morrison et al., Inorg. Chem. 1979, 18, 1971. Solutions of superoxide ion in aprotic solvents have been prepared using electrochemical means. Sawyer et al., Anal. Chem. 1982, 54, 1720. The superoxide ion used for degrading halogenated hydrocarbons in U.S. Pat. Nos. 4,468,297 and 4,410,402 is generated in a controlled potential electrolysis cell which uses aprotic solvent for the electrolyte.
The known methods for generating superoxide ion have several disadvantages. Methods for generating superoxide ion based on pulse radiolysis, photolysis, or electrolysis, all require radiation or electrical energy sources. Typically, the energy costs for these methods are prohibitively high, especially for applications such as degrading halogenated hydrocarbons on an industrial scale. Likewise, methods for generating superoxide ion based on decomposing hydrogen peroxide are prohibitively expensive for many applications due to the cost of hydrogen peroxide. Consequently, other methods for generating superoxide ion are desired.
The art has thus not solved the problem of efficient degradation and detoxification of halogenated organic compound wastes. In particular, while methods employing liquid phase reagents containing superoxide ion have shown some promise, methods known to the art are technically difficult, expensive or impractical to use in industrial and soil-remediation projects.